It may very well be that the person uploading the videos and subtitles on YT is the same person that "translated" on Viki. You will never know unless you ask them directly. Although he/she doesn't own rights for the video, they might own the subtitles. At which point, whether the subs are on Viki or YT, it doesn't matter.
Did the same user translate the entire series, or just parts of it? Are all subs from YT the same as on Viki?
Old projects have a lot of (to us now) weird combos and customs in translating.